Hi,
this is really not much information. First of all before you launch your configuration make sure that you have no compilation problems in the relevant bundles. That's what the dialog tells you.

In order to use classes from a library located in bundle A you need to add the lib to the classpath of that bundle. Go to the Runtime Tab of the PDE-Editor (plugin.xml)and add the lib to the classpath. Then decide which packages of that lib you want to export for other plugins. Add these packages to the "Exported Packages" section located on the same editor page (this might be what you have missed out).

You can now import these packages in a bundle B by either specifying Bundle A as "Required Bundle" on the dependency tab or by importing packages of Bundle A in the "Imported Packages" section of the same tab page.

Of course there could be all kinds of other compilation problems in your project.